The first attempt at making an obsidian roof at spawn was done by the group Obsidian Sky. Their reason for doing this was so that it will make spawn more difficult by having newfags spawn under the roof, thus making it dark and harder to see for vanilla players without cheat clients. Eventually the Obsidian Sky group got bored, and only completed a 500 x 500 square of the spawn region instead of a larger radius.
The second attempt at this project was by a group called "Project Obsidian Roof," founded by c0mmie_. Not only did they want to make an obsidian roof, they also planned to sponge out all water within the roof and on the edges, and create a large trench filled with lava and withers. Unfortunately, c0mmie_'s main dupe stash was griefed after the group was founded this led to the discord server to slowly become inactive.
Final and successful attempt
The final and successful attempt to create an obsidian roof over spawn was started by the Infinity Incursion. This attempt utilized a recently added "autobuild" module in Salhack which allowed semi-automatic platform building. Members of the II got to work, and within a few weeks, the roof was created.